Past experience with ionization U.V. detectors has shown that as the detector ages, especially when it is operating under high temperature conditions such as .congruent. 450.degree. F., it becomes subject to a thermal runaway condition which falsely indicates the presence of U.V. when none is actually present. It has further been known that by periodically or cyclically reversing the voltage on the detector, in other words by reducing the average voltage applied to the detector, the effective life of the detector tube can be greatly extended. The reversing of the voltage has been implemented in the past by the use of a special AC supply such as is shown in the Engh U.S. Pat. No. 3,435,225, assigned to the same assignee as the present invention, although the main purpose was for extinguishing the tube, and also by the use of a constantly running multivibrator in the power supply which continually switches the potential applied to the detector from a normal forward potential to a lower reverse potential.